Multi-tone amplitude photomask

ABSTRACT

A method of fabricating a multi-tone amplitude photomask includes providing a mask substrate. The method includes providing a stepped pattern in at least one layer of material on a surface of the mask substrate. The stepped pattern includes at least two steps and at least three levels. Each level of the stepped pattern provides a different intensity of light when a light source shines light on the stepped pattern.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/442,244, filed Jun. 14, 2019, and titled “Multi-Tone AmplitudePhotomask,”, which is a continuation of U.S. patent application Ser. No.15/641,941, filed Jul. 5, 2017, and titled “Multi-Tone AmplitudePhotomask,” which is a continuation of PCT Application No.PCT/US2016/012242, filed Jan. 5, 2016, and titled “Multi-Tone AmplitudePhotomask,” which claims priority to U.S. Provisional Application No.62/100,062, filed Jan. 5, 2015, and titled “Multi-Tone AmplitudePhotomask,” each of the foregoing being incorporated by reference intheir entireties.

FIELD

This patent application generally relates to a way of providing amulti-tone photomask. More particularly, it relates to a scheme formaking a photomask that has multiple levels of optical density. Evenmore particularly, it relates to a scheme for making a photomask thatprovides a pattern on a workpiece with several different heights ofphotoresist.

BACKGROUND

Multilayer structures have been formed on substrates with multiplemasking steps. Each mask level has been aligned to a previous masklevel, introducing alignment error. So each wafer may have slightlydifferent alignments from others. Thus, products formed on differentwafers will have different misalignments and will be different from eachother. In addition, each masking step has required a substantial amountof time, and multiple masking steps have required a multiple of thisprocessing time for manufacturing the products. In addition, each maskand each masking step has a substantial cost. Applicants recognized thatbetter schemes than those available are needed, and such solutions areprovided by the following description.

SUMMARY

One aspect of the present patent application is a method of fabricatinga multi-tone amplitude photomask. The method includes providing a masksubstrate. The method also includes providing a stepped pattern in atleast one layer of material on a surface of said mask substrate. Thestepped pattern includes at least two steps and at least three levels.Each level of the stepped pattern provides a different intensity oflight when a light source shines light on the stepped pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following detailed description,as illustrated in the accompanying drawings, in which:

FIGS. 1A-1F are cross sectional views showing one embodiment of aprocess for fabricating a multi-tone amplitude photomask of the presentpatent application with 2 layers of material and 3 levels;

FIGS. 2A-2B are cross sectional views of a process of using themulti-tone amplitude photomask of FIG. 1F to expose and developphotoresist on a workpiece to provide a three-dimensional pattern, suchas a staircase, stepped pyramid, or stepped cone pattern in thephotoresist with 3 levels;

FIG. 3A is a graph showing transparency v. wavelength for variousthicknesses of nickel and chromium respectively;

FIG. 3B is a graph showing developed photoresist depth v. exposureenergy;

FIGS. 4A-4E are cross sectional views showing an embodiment of a processfor fabricating a multi-tone amplitude photomask of the present patentapplication with 3 layers of material and 4 levels;

FIGS. 5A-5B are cross sectional views of a process of using themulti-tone amplitude photomask of FIG. 4E to expose and developphotoresist on a workpiece to provide a three-dimensional pattern, suchas a staircase, stepped pyramid, or stepped cone pattern in thephotoresist with 4 levels;

FIG. 6A is a three-dimensional view of a multi-tone amplitude photomaskwith 6 levels formed by the process of the present patent application ina stepped cone pattern and the 6 levels formed in photoresist on thesubstrate in a stepped cone pattern using this photomask;

FIG. 6B is a three-dimensional view of a multi-tone amplitude photomaskwith 4 levels formed by the process of the present patent application ina diffractive lens pattern and the 4 levels formed in photoresist on thesubstrate in a diffractive lens pattern using this photomask;

FIG. 6C is a three-dimensional view of a multi-tone amplitude photomaskwith 7 levels formed by the process of the present patent application ina diffraction grating pattern and the 7 levels formed in photoresist onthe substrate in a diffraction grating pattern using this photomask;

FIGS. 7A-7G are cross sectional views showing an embodiment of a processfor fabricating a multi-tone amplitude photomask of the present patentapplication in which top and bottom layers are the same material with anetch-stop layer there between;

FIGS. 8A-8D are cross sectional views showing the use of a multi-toneamplitude photomask of the present patent application to fabricate anarray of aerosol nozzles;

FIGS. 9A-9D are cross sectional views showing the use of a multi-toneamplitude photomask of the present patent application to fabricate anarray of micro lenses.

FIG. 10A is a cross sectional view showing laser ablation of a lightabsorbing polymer on a mask substrate to form a stepped pattern in thelight absorbing polymer;

FIG. 10B is a cross sectional view showing use of a holographic mask forlaser ablation of a light absorbing polymer on a mask substrate to forma stepped pattern in the light absorbing polymer; and

FIG. 11 is a cross sectional view showing an additive process to form astepped pattern in the light absorbing polymer using a printer thatprovides liquid polymer droplets.

DETAILED DESCRIPTION

The present applicants created methods of making and using a multi-toneamplitude photomask. The multi-tone amplitude photomask has multiplelevels of optical density. Developing photoresist on a workpiece afterexposure using the mask results in a patterned photoresist with athree-dimensional structure, which is a pattern with several differentheights of photoresist on the workpiece.

In one embodiment, to fabricate a multi-tone amplitude photomask, a masksubstrate is provided and a stepped pattern is provided in at least onelayer of material on a surface of the mask substrate. The steppedpattern includes at least two steps and at least three levels. Eachlevel of the stepped pattern provides a different level of intensity oflight when measured at the mask when a light source shines light on thestepped pattern.

In one embodiment, the multiple levels of optical density on themulti-tone amplitude photomask are produced by providing multiple layersof light attenuating materials. Each of the layers is patternedindividually. Each layer has an attenuation or optical density thatdepend on material characteristic and thickness. A photomask having twolayers of light attenuating materials will produce 3 levels ofattenuation of incident light for the exposure: no attenuation whereboth layers have been etched, the attenuation because of the opticaldensity of the bottom layer in areas where the top layer has been etchedand attenuation because of the optical density of both layers combinedwhere neither layer has been etched.

More layers of light attenuating material can be provided and theprovision of more layers of light attenuating material on the substratewill allow more levels of attenuation and more different heights in theexposed and developed photoresist on the workpiece.

In one embodiment, a two layer structure, including at least one layerof a partially transparent material is blanket deposited on atransparent substrate to begin the fabrication of the 2-layer multi-toneamplitude photomask, as shown in FIG. 1A. In this embodiment, bottomlayer 20 b is a different material and has a different etch chemistrythan top layer 20 a so that when top layer 20 a is etched, bottom layer20 b is not. In one example, top layer 20 a is nickel and bottom layer20 b is chromium. Mask substrate 24 is a transparent material, such asglass, quartz, or fused silica. Although described in terms of partialtransparency for a reflection mask, it is understood that the process isthe same for fabricating a transmission and for fabricating a reflectionmask, and in the case of a reflection mask the at least one layer is ofa partially reflective material.

The light dose reaching photoresist on a workpiece through both layers20 a and 20 b depends on the thicknesses of layers 20 a and 20 b. In oneembodiment, bottom layer 20 b is about 5 nm thick to provide about halfthe applied UV dose penetrating through bottom layer 20 b when thisbottom layer is alone on mask substrate 24. Substantially less UV lightpasses through in regions having both top layer 20 a and bottom layer 20b depending on the thickness of top layer 20 a. For example, if toplayer 20 a is 5 nm thick as well, as shown in the graphs in FIG. 3A,transparency for light going through both layers is reduced to about30%. Top layer 20 a can be thicker to permit less light to pass throughboth layers and to reduce the exposure of the photoresist in regionsunder both layers. For example, top layer can be in the range from 5 nmto 100 nm thick. Other thicknesses of nickel and chromium can be used totailor vertical dimensions of the photoresist after development on aworkpiece exposed using this mask. For example, top layer can be in therange from 5 nm to 300 nm.

Alternatively to thin metal layers, dielectric materials, such assilicon dioxide and silicon nitride, titanium dioxide, hafnium oxide,and tantalum pentoxide can be used with thicknesses tailored to providepartial transparency or partial reflectivity to a predeterminedwavelength of light that will be used with the mask. For improvedtransmittivity or reflectivity multiple layers of high and low indexmaterials are used. For example, while a thickness equal to a quarterwave in the material would provide maximum transmission through adielectric layer and while a thickness equal to a half wave in thematerial would provide maximum reflection, a thickness adjusted betweena quarter wave and a half wave would provide partial transmission andpartial reflection. In addition to metals and dielectrics, otherpartially transparent materials can be used, such as dyed polymer.

Top and bottom metal or dielectric layers 20 a and 20 b can be depositedby techniques such as evaporation, chemical vapor deposition,sputtering, and physical vapor deposition. For a dyed material, thickerlayers can be deposited and spinning, spraying, or roll-on depositioncan be used.

Once top and bottom layers 20 a and 20 b have been deposited, firstlayer of resist 26 is deposited, as shown in FIG. 1B and patterned toform first openings 28 in patterned first layer of resist 26, as shownin FIG. 1C. In one embodiment, first layer of resist 26 is photoresistand the patterning to form first openings 28 is with UV light through amask followed by developer. Other techniques, such as laser writing orelectron beam writing of the pattern in the first layer of resist 26 canbe used for the exposure. X-ray exposure of photoresist 26 with a maskcan also be used. Alternatively, laser writing with a beam to ablate alight absorbing polymer, such as polymethyl methacrylate (PMMA) with aUV absorbing dye, or a photoresist, to a desired depth can also be used,as shown in FIG. 10A. Alternatively, a holographic mask can be used witha laser to provide ablating a stepped pattern in a light absorbingpolymer on a mask substrate, as shown in FIG. 10B. In anotheralternative, a process such as stencil screen printing or roll-onmasking material can be used to provide the patterned first layer ofresist during its deposition.

Top layer of material 20 a is etched away in conformance to firstopenings 28 in first layer of resist 26, as also shown in FIG. 1C. Inone embodiment, top layer of material 20 a is nickel, and is etched awaywith Transene Nickel Etch TFG, available from Transene Company, Inc.,Danvers, Mass., in first openings 28 in first layer of resist 26 to formtop layer openings 30 in top layer of material 20 a. As Transene NickelEtch TFG will not etch chromium of bottom layer of material 20 b underthe nickel, bottom layer of material 20 b remains fully in place, asalso shown in FIG. 1C.

First layer of resist 26 is now stripped off and a second layer ofresist is deposited and patterned to form second openings 38 inpatterned second layer of resist 36, as shown in FIG. 1D. In thisembodiment, second openings 38 in patterned second layer of resist 36are within top layer openings 30 in top layer of material 20 a, as alsoshown in FIG. 1D. In one embodiment, second layer of resist 36 isphotoresist and the patterning to form second openings 38 includesaligning to alignment marks and exposing with UV light through a maskfollowed by developer. Other techniques, such as laser writing orelectron beam writing of the pattern in the second layer of resist 36can be used for the exposure. X-ray exposure of photoresist 36 with amask, can also be used. Alternatively, laser writing with a beam toablate a light absorbing polymer, such as resist, to a desired depth canbe used. In another alternative, a process such as stencil screenprinting, roll-on masking material, and ink-jet printing can be used toprovide the patterned second layer of resist aligned with the firstlayer.

As shown in FIG. 1E, bottom layer of material 20 b is now etched inconformance to second openings 38 in second layer of resist 36. In oneembodiment, bottom layer of material 20 b is chromium and is etched awaywith Transene Chromium Etch TFE in second openings 38 in second layer ofresist 36 to form bottom layer openings 40 in bottom layer of material20 b. As Transene Chromium Etch TFE will not etch nickel of top layer ofmaterial 20 a, and as nickel layer 20 a is also protected by secondlayer of resist 36 in this embodiment, top layer of material 20 aoutside of first openings 30 remains fully in place, as shown in FIG.1E.

Finally, 2-layer multi-tone amplitude photomask 50 is completed whensecond photoresist 36 is stripped off, leaving three regions of masksubstrate 24. Regions 42 of mask substrate 24 have neither bottom layerof material 20 b nor top layer of material 20 a; regions 44 of masksubstrate 24 have bottom layer of material 20 b but do not have toplayer of material 20 a, and regions 46 of mask substrate 24 have bothbottom layer of material 20 b and top layer of material 20 a, as shownin FIG. 1F. In certain regions, bottom layer of material 20 b and toplayer of material 20 a are arranged in staircase, stepped pyramid, orstepped cone shaped pattern 48 on mask substrate 24 while in otherregions bottom layer 20 b is by itself.

Staircase, stepped pyramid, or stepped cone shaped pattern 48 inmulti-tone amplitude photomask 50 is reproduced in photoresist onworkpiece 60 with a mask exposure, as shown in FIGS. 2A-2B. First,workpiece 60, such as a semiconductor wafer, is coated with workpiecephotoresist 61, as shown in FIG. 2A, and exposed to UV light shiningthrough multi-tone amplitude photomask 50. Photoresist 61 has athickness many times greater than the thickness of staircase, steppedpyramid, or stepped cone shaped pattern 48 in multi-tone amplitudephotomask 50. In one embodiment, photoresist 61 is about 1 micron thick,which is about 100 times the full thickness of metal layers on photomask50. Photoresist 61 typically has a thickness in the range from 0.5 to 3microns and can be in the range from 0.1 to 100 microns and even up tomillimeters thick.

A technique, such as contact printing or providing multi-tone amplitudephotomask 50 in a projection aligner, a stepper, or a projection scanneris used for exposing photoresist layer 61 on workpiece 60 with photomask50.

As both transmittivity and reflectivity are dependent on metal thicknesson photomask 50, photoresist layer 61 on workpiece 60 can be exposedeither in transmission or reflection mode. If dielectric coatings orstacks of dielectric coatings or different metals with differentinherent reflectivities are used on photomask 50 reflection mode canalso be used.

In addition to UV light for exposing photoresist, other wavelengths canbe used for exposing a photosensitive layer on a workpiece usingphotomask 50, including X-ray, visible, and infrared. Light sources,such as a mercury or xenon lamp, a laser, or an X-ray machine can beused.

If laser or electron beam writing is used to provide the patternedlayers of material in the multi-tone amplitude photomask, maskgeneration can take many hours. However, the multi-tone amplitudephotomask, once generated, can be used many times over to quickly exposephotoresist on many workpieces, saving a great deal of processing timefor creating a desired three-dimensional pattern in photoresist on theworkpieces. Applicant found that processing time for exposingphotoresist on each workpiece with the multi-tone amplitude photomaskwas in the range of minutes as compared to the prior art process oflaser writing photoresist on each workpiece to create athree-dimensional pattern in the photoresist, which took many hours.

Regions 42 on mask substrate 24 with neither bottom layer of material 20b nor top layer of material 20 a allow the UV light used in themulti-tone amplitude photomask exposure to strike photoresist 61 onworkpiece 60 unimpeded. Regions 44 of mask substrate 24 with bottomlayer of material 20 b but not top layer of material 20 a allow enoughlight through material 20 b to sufficiently expose a top portion ofphotoresist 61 on workpiece 60. Regions 46 of mask substrate 24 withboth bottom layer of material 20 b and top layer of material 20 a do notallow enough light through both layers of material 20 a and 20 b toexpose any substantial portion of photoresist 61 on workpiece 60.

Thus, after developing, a pattern in mask 50 with three distinct heightsof material in regions 42, 44, and 46 on mask substrate 24, isreproduced in photoresist 61 on workpiece 60 as photoresist regions 62,64, and 66, such as staircase, stepped pyramid, or stepped cone shapedpattern 68 with its three heights, as shown in FIG. 2B: zero photoresistin region 62, about half the photoresist remaining in region 64, andfull height photoresist in region 66.

In one experiment, AZ 4330 photoresist, Clariant Corporation AZElectronic Materials, Somerville N.J., was spin applied to semiconductorwafer workpieces. The workpieces with 3 microns of photoresist werepre-exposure baked at 105° C. for 15 minutes. They were exposed throughthe multi-tone amplitude photomask using an Oriel mask aligner with a400 Watt mercury lamp for about 30 seconds to achieve about 100 mJ/cm².The workpieces were developed in AZ 400K diluted 3:1 for 60 seconds.Resulting photoresist topology was measured on a KLA/Tencor P-2 stylusprofilometer.

A graph showing developed photoresist depth as a function of exposureenergy with the AZ 4330 photoresist under these bake, exposure, anddevelop conditions is provided in FIG. 3B. The graph shows that the top0.5 micron of resist is developed out when the dose is about 38 mJ/cm²,1.0 microns when the energy dose is about 50 mJ/cm², 1.5 micron when theenergy dose is about 60 mJ/cm², 2.0 microns when the energy dose isabout 72 mJ/cm², 2.5 microns when the energy dose is about 84 mJ/cm²,and 3.0 microns when the energy dose is about 100 mJ/cm².

In another embodiment, 3 layers of material are formed on a transparentsubstrate, as shown in FIGS. 4A-4E, and the resulting four exposurelevels are similarly transferred to photoresist on a workpiece with amask exposure, as shown in FIGS. 5A-5B, to make a 4-levelthree-dimensional structure in the photoresist. In this embodiment the3-layer structure includes at least two layers of partially transparentmaterial or at least two layers of partially reflective material. The 3layers of partially transparent or reflective material are deposited asblanket layers on a transparent mask substrate to begin the fabricationof the multi-tone amplitude photomask, as shown in FIG. 4A. In oneexample, top layer 70 a is copper, middle layer 70 b is nickel, bottomlayer 70 c is chromium, and mask substrate 74 is glass, quartz, or fusedsilica. Bottom layer 70 c is 2 or 3 nm thick to provide partialtransparency to UV light when this bottom layer is alone on masksubstrate 74. Middle layer is also 2 or 3 nm thick to still providepartial transparency to UV light when both middle layer 70 b and bottomlayer 70 c are on mask substrate 74.

In the reflection mode, this same construction of metal layers wouldoperate in a reverse manner with the substrate reflecting the leastlight onto the workpiece and the multiple layers above it reflecting themost. It is also possible for the stack of coatings to be absorbing andto be placed on a reflective substrate such as aluminum or an aluminumcoated fused silica substrate such that the greatest reflection wouldcome from the substrate layer, with each successive layer above itabsorbing more light and thus reflecting less light.

Top layer is sufficiently thick so very little UV light passes throughin regions having all three layers or so insufficient light passesthrough all three layers to reach the threshold exposure needed by thephotoresist used on the workpiece for any photoresist to be developedout. For example, top layer 70 a is 10 or 20 nm thick. Top layer 70 acan be thicker. Other thicknesses of copper, nickel, and chromium can beused to tailor vertical dimensions of the photoresist on a workpieceexposed using this mask to provide a desired three-dimensional structurein the photoresist.

As described herein above, other partially absorbing materials can beused, such as dielectric layers or dye containing polymer layers. Top,middle, and bottom partially transmitting layers 70 a, 70 b, and 70 care each deposited on mask substrate 74 by a technique, such asevaporation, chemical vapor deposition, sputtering, and physical vapordeposition.

Once top, middle, and bottom layers 70 a, 70 b, and 70 c have beendeposited on mask substrate 74, first layer of resist 76 is depositedand patterned to form first openings 78 in patterned first layer ofresist 76, as shown in FIG. 4B. In one embodiment, first layer of resist76 is photoresist and the patterning to form first openings 78 in firstlayer of resist 76 is with UV light through a mask followed bydeveloper. Other exposure techniques, such as laser or electron beamwriting exposure or X-ray exposure of the resist can be used.Alternatively, a process such as stencil screen printing or roll-onmasking material is used to provide the patterned first layer of resistduring its deposition.

Top layer of material 70 a is etched away in conformance to firstopenings 78 in first layer of resist 76, as also shown in FIG. 4B. Inthe embodiment in which top layer of material 70 a is copper, it may beetched away with Transene Copper etch APS-100. Thus, top layer openings80 in top layer of material 70 a are formed while the copper etchantdoes not affect nickel of middle layer of material 70 b under thecopper, so middle layer of material 70 b and bottom layer 70 c remainfully in place, as also shown in FIG. 4B.

First layer of resist 76 is now stripped off and second layer of resist86 is deposited and patterned to form second openings 88 in patternedsecond layer of resist 86, as shown in FIG. 4C. The patterning to formsecond openings 88 in second layer of resist 86 is with UV light througha mask, using laser or electron beam writing exposure or X-ray exposureof second layer of resist 86. Alternatively, a process such as stencilscreen printing or roll-on masking material is used to provide thepatterned second layer of resist during its deposition. In thisembodiment, second openings 88 in patterned second layer of resist 86are within top layer openings 80 in top layer of material 70 a, as alsoshown in FIG. 4C.

Middle layer of material 70 b is etched in conformance to secondopenings 88 in second layer of resist 86, as also shown in FIG. 4C. Inthe embodiment in which middle layer of material 70 b is nickel and isetched away with Transene Nickel Etch TFG in second openings 88 insecond layer of resist 86 to form middle layer openings in middle layerof material 70 b. As the Transene nickel etchant will not etch chromiumof bottom layer of material 70 c, and as copper layer 70 a is protectedby second layer of resist 86, top layer of material 70 a outside offirst openings 80 remains fully in place and bottom layer of material 70c remains fully in place, as shown in FIG. 4C.

Second layer of resist 86 is now stripped off and a third layer ofresist 96 is deposited and patterned to form third openings 98 inpatterned third layer of resist 96, as shown in FIG. 4D. The patterningto form third openings 98 in third layer of resist 96 is with UV lightthrough a mask, using laser or electron beam writing exposure or X-rayexposure of second layer of resist 96. Alternatively, a process such asstencil screen printing or roll-on masking material is used to providethe patterned second layer of resist during its deposition. In thisembodiment, third openings 98 in patterned third layer of resist 96 arewithin middle layer openings 88 in middle layer of material 70 b, asalso shown in FIG. 4D.

Bottom layer of material 70 c is etched in conformance to third openings98 in third layer of resist 96, as also shown in FIGS. 4D and 4E. In theembodiment in which bottom layer of material 70 c is chromium, it isetched away with Transene Chromium Etch TFE in third openings 98 inthird layer of resist 96 to form bottom layer openings 102 in bottomlayer of material 70 c extending to mask substrate 74.

Finally, multi-tone amplitude photomask 120 a is completed when thirdphotoresist 96 is stripped off, leaving four regions on mask substrate74, as also shown in FIG. 4E. Openings 102 on mask substrate 74 haveneither bottom, middle, or top layer of material 70 a, 70 b, and 70 c;regions 104 on mask substrate 74 have only bottom layer of material 70c; regions 106 on mask substrate 74 have only bottom layer of material70 c and middle layer of material 70 b; regions 108 on mask substrate 74have all three: bottom, middle, and top layers 70 a, 70 b, 70 c, asshown in FIG. 4E. In certain regions, the three layers of material 70 bare arranged in stepped pattern 110 on mask substrate 74. When viewed inthree dimensions, stepped pattern 110 can be a staircase, a steppedpyramid, or a stepped cone.

Stepped pattern 110 provided in 3-layer multi-tone amplitude photomask120 a is reproduced in photoresist on workpiece 130, as shown in FIGS.5A-5B. First, workpiece 130, such as a semiconductor wafer, is coatedwith workpiece photoresist 131, as shown in FIG. 5A, and exposed to UVlight shining through 3-layer multi-tone amplitude photomask 120 a. Atechnique, such as contact printing or providing 3-layer multi-toneamplitude photomask 120 a in a projection aligner or scanner is used forthe exposure.

Openings 102 on mask substrate 74 with neither bottom, middle, or toplayer of material 70 c, 70 b, and 70 a allow the exposure light, such asUV light, to strike photoresist 131 on workpiece 130 unimpeded. Regions104 of mask substrate 74 with bottom layer of material 70 c but notmiddle or top layer of material 70 b and 70 a allow enough light throughbottom layer material 70 c to sufficiently expose a top portion ofphotoresist 131 on workpiece 130. Regions 106 of mask substrate 74 withboth bottom and middle layers of material 70 c and 70 b allow less lightthrough but still allow enough for an exposure closer to the top surfaceof resist 131. Regions 108 of mask substrate 74 with all three, bottom,middle and top layers of material 70 c, 70 b, and 70 a have enoughmaterial blocking light to prevent any part of photoresist 131 onworkpiece 130 from being sufficiently exposed so developer does notremove material. Thus, upon developing, a pattern with four distinctheights 142, 144, 146, and 148 is formed in photoresist 131 on workpiece130, as shown in FIG. 5B to create a three-dimensional structure: zerophotoresist in region 142, about one third height photoresist in region144, about two thirds height photoresist in region 146 and full heightphotoresist in region 148.

More thin metal layers can be formed on transparent mask substrate 74,as shown with 5-layer multi-tone amplitude photomask 120 a in thethree-dimensional view in FIG. 6A. In one embodiment, as the number oflayers 70 a′, 70 b′, 70 c′, 70 d′, and 70 e′ increases, the thickness ofeach layer is decreased so some light can get through the bottom four ofthem. In one embodiment, the full thickness of all layers combined 70 a′and 70 b′ and 70 c′ and 70 d′ and 70 e′ is sufficient to effectivelyblock exposure of resist on a workpiece while regions with step-wisefewer layers, such as the annular region having layers 70 d′ and 70 e′,allow correspondingly greater doses of light. With the 3-layermulti-tone amplitude photomask 120 a in FIG. 5B, this causes a deeperexposure in photoresist 131 on mask substrate 74 sufficient fordeveloping a deeper three-dimensional pattern. Similarly, for 5-layermulti-tone amplitude photomask 120′, as shown in FIG. 6A, a five-levelpattern is produced. Thus, the position and number of layers inmulti-tone amplitude photomask 120, 120′ is reproduced in thephotoresist after exposure and development.

In FIG. 6A, openings in photoresist are round, and the largest roundopening in resist is for etching top metal layer 70 a′. For eachsubsequent layer of metal down 70 b′, 70 c′, 70 d′, 70 e′ the opening inthe corresponding layer of resist is smaller, to leave metal layers inplace with increasingly larger diameter. This scheme in one dimension isalso shown in the cross sections of FIGS. 4B-4E for the 3-layermulti-tone amplitude photomask fabrication process.

Other embodiments can have different patterns for the metal layers, suchas those multi-layer multi-tone amplitude photomask shown fordiffractive lens 154 in FIG. 6B and for diffraction grating 155 in FIG.6C.

In one embodiment, and as shown in FIG. 6B, photoresist 156 forfabricating a diffractive lens is 3 microns tall and each step 157 inphotoresist 156 is 1 micron high for focusing light in the infraredrange. In another embodiment, an array of 400 of these diffractivelenses was formed, the whole array 20 mm in diameter. Each photoresistlenslet in the array was 500 microns across and 1.7 um tall for focusinglight in the near infrared wavelength, 0.85 microns. To fabricate thediffractive lens a directional reactive ion etch was used on photoresist156 on fused silica workpiece 158 to replicate the photoresist patternin the fused silica below. In one embodiment, the etch chemistry isadjusted to provide the etch rate approximately the same for photoresistand fused silica. Parameters for such control include oxygen andfluorine precursor gas flow rates and pressures.

To fabricate multi-layer multi-tone amplitude photomask 155 for thediffraction grating with the staircase shape of FIG. 6C, a rectangularopening in each successive resist layer is used in its fabrication. Thelargest rectangular opening in the first resist applied leaves resistcoating only top metal layer 70 a″ across its width, allowing etchingtop layer 70 a″ over all other anticipated steps. Once top layer 70 a″is etched except under the first resist, that resist is stripped and asecond layer of resist is deposited for coating both top metal layer 70a″ and second metal layer 70 b″ and for allowing etching second metallayer 70 b″ over all other anticipated steps. The process continues withlayers of resist with increasing coating area and decreasing openingsize and successively etching metal layers 70 c″, 70 d″, 70 e″, and 70f′ to create staircase mask 155. As described herein above, differentmetals with different etch characteristics and different etchants areused for the different layers. Alternatively, different dielectrics ordyed polymers and different etchants are used for the different layers.

As with the diffractive lens, the diffraction grating pattern formed inphotoresist is replicated in the fused silica workpiece using reactiveion etch. The diffraction grating so formed spreads light striking at anangle into component colors. One application is for use in aspectrometer. In one embodiment, the height of each staircase pattern isin the range from hundreds of nm to tens of microns, the height beingrelated to wavelength of light to be used. Applicant found that thefabrication process described herein above enables control over thewidth of each stair, the angle of the staircase, the number of stairs inthe staircase, and ratio of width to depth in the staircase.

Another embodiment includes an etch stop layer in the multi-toneamplitude photomask, allowing the top and bottom layers to be the samematerial, as shown in FIGS. 7A-7G. In this embodiment, top and bottomlayers of material 160 a, 160 b are blanket deposited, sandwiching etchstop layer 163 on transparent substrate 164 to begin the fabrication ofthe multi-tone amplitude photomask, as shown in FIG. 7A. In one example,top and bottom layers 160 a, 160 b are both chromium, etch stop layer163 is silicon dioxide, and substrate 164 is glass, quartz, or fusedsilica. Bottom layer 160 b is about 5 nm thick to provide partialtransparency to UV light when this bottom layer is alone on substrate164. Very little UV light passes through in regions having both toplayer 160 a and bottom layer 160 b if top layer 160 a is at least 10 nmthick. Top layer 160 a can be thicker. Other thicknesses of chromium canbe used to tailor vertical dimensions of the photoresist on a workpieceexposed using this mask. Other partially absorbing materials can beused, such as nickel or copper. Top and bottom layers 160 a and 160 bcan be deposited by techniques, such as evaporation, chemical vapordeposition, sputtering, and physical vapor deposition, etch stop layer163 can be deposited using chemical vapor deposition.

While a 2-layer structure is shown in FIGS. 7A-7D, a structure with morelayers can also be fabricated using this process repetitively with anetch stop layer between each metal layer. Applicants found that a layerof silicon dioxide 4 nm thick is sufficient to provide etch stopping.

Once top and bottom layers 160 a and 160 b and etch stop layer 163 havebeen deposited, first layer of resist 166 is deposited, as shown in FIG.7B and patterned to form first openings 168 in patterned first layer ofresist 166, as shown in FIG. 7C. In one embodiment, first layer ofresist 166 is photoresist. The patterning to form first openings 168 infirst layer of resist 166 is with UV light through a mask, using laseror electron beam writing exposure or X-ray exposure, followed bydeveloper. Alternatively, a process such as stencil screen printing orroll-on masking material is used to provide patterned first layer ofresist 166 during its deposition.

Top layer of material 160 a is etched away in conformance to firstopenings 168 in first layer of resist 166, as also shown in FIG. 7C. Inone embodiment, top layer of material 160 a is chromium, and is etchedaway with Transene Chromium Etch TFE in first openings 168 in firstlayer of resist 166 to form top layer openings 170 in top layer ofmaterial 160 a. As Transene Chromium Etch TFE will not etch silicondioxide of etch stop layer 163, both etch stop layer 163 and bottomlayer of material 160 b remain fully in place, as also shown in FIG. 7C.

First layer of resist 166 is now stripped off and a second layer ofresist is deposited and patterned to form second openings 178 inpatterned second layer of resist 176, as shown in FIG. 7D. Secondopenings 178 in patterned second layer of resist 176 are within toplayer openings 170 in top layer of material 160 a, as shown in FIGS. 7Cand 7D. In one embodiment second layer of resist 176 is photoresist. Thepatterning to form second openings 178 in second layer of resist 176 iswith UV light through a mask, using laser or electron beam writingexposure or X-ray exposure, followed by developer. Alternatively, aprocess such as stencil screen printing or roll-on masking material isused to provide patterned second layer of resist 176 during itsdeposition.

Etch stop 163 is now etched in conformance to second openings 178 insecond layer of resist 176, as shown in FIG. 7E. In one embodiment, etchstop layer 163 is silicon dioxide and is etched away with hydrofluoricacid in second openings 178 in second layer of resist 176 to formopenings 180 in etch stop layer 163. As hydrofluoric acid will not etchchromium of bottom layer of material 160 b, and as chromium layer 160 ais also protected by second layer of resist 176, neither layer ofchromium is affected by the oxide etch, as shown in FIG. 7E.

Bottom layer 160 b is now etched in conformance to second openings 178in second layer of resist 176, as shown in FIG. 7F. In one embodiment,bottom layer 160 b is chromium and is etched away with Transene ChromiumEtch TFE in second openings 178 in second layer of resist 176 to formopenings 180 in etch stop layer 163. As Transene Chromium Etch TFE willnot etch the glass of substrate 164 and as chromium layer 160 a is alsoprotected by second layer of resist 176, nothing else is affected by theTransene Chromium Etch TFE, as shown in FIG. 7F.

Finally, multi-tone amplitude photomask 190 is completed and ready foruse when second photoresist 176 is stripped off, leaving three regionsof substrate 164. Regions 192 of substrate 164 have neither layer ofmaterial 160 a and 160 b nor etch stop layer 163; regions 194 ofsubstrate 164 have bottom layer of material 160 b and etch stop layer163 but do not have top layer of material 160 a, and regions 196 ofsubstrate 164 have both layers of material 160 a and 160 b and etch stoplayer 163, as shown in FIG. 7G. In certain regions, bottom layer ofmaterial 160 b with etch stop layer 163 and top layer of material 160 aare arranged in staircase pattern on substrate 164.

Multi-tone amplitude photomask 50, 120, 120′, 190 are used to fabricatedevices that have layered or curved shapes. In one embodiment, 3-layermulti-tone amplitude photomask 120 is used in the fabrication of anarray of aerosol nozzles, as shown in FIGS. 8A-8D. In anotherembodiment, 3-layer multi-tone amplitude photomask 120 a is used in thefabrication of an array of micro lenses, as shown in FIGS. 9A-9D.

For fabricating the array of aerosol nozzles, workpiece 220, coated withelectroplating seed layer 222, and provided with blanket photoresistlayer 224, is exposed to ultraviolet light through 3-layer multi-toneamplitude photomask 120, as shown in FIG. 8A. After developingphotoresist 224 has 4-step staircase structures 226, with cavities 228there between, as shown in FIG. 8B. In one embodiment the lowest step in4-step photoresist staircase structure 226 is 0.5 microns thick, thesecond step is 0.5 microns thick, and the highest step is 4 micronsthick. The highest step has a diameter that will become the orifice ofthe nozzle. In one embodiment, electroplating seed layer 222 issputtered nickel with a thickness in the range from about 5 nm to about500 nm on sputtered chromium with a thickness in the range from 1 to 2nm. The seed layer is allowed to oxidize to form a passivation thatallows later release of electroplated material from seed layer 222. Seedlayers can also be provided on workpiece 220 using evaporation, ion beamdeposition, physical vapor deposition, and laser deposition.

Workpiece 220 is now electrically connected for electroplating throughplating seed layer 222 as masked by 4-step staircase photoresiststructures 226, as shown in FIG. 8C, and electroplated palladium nickelalloy fills cavities 228 between 4-step staircase photoresist structures226. Other noble metals and alloys including noble metals can be used.

Photoresist 224 and 226 are now removed, such as in an acetone bath,leaving array of electroplated palladium nickel alloy micro nozzles 230which are released from substrate 220 mechanically, by peeling, as shownin FIG. 8D. The process allows each nozzle of the array of nozzles tohave an orifice defined by the highest step in photoresist structure 226having a diameter in the range from 2.5 to 4.5 microns.

For fabricating the array of micro lenses, workpiece 250 is providedwith blanket photoresist layer 254, and is exposed to ultraviolet lightthrough 3-layer multi-tone amplitude photomask 120, as shown in FIG. 9A.For the lenses, workpiece 250 is fabricated of a material, such as fusedsilica. After developing, photoresist 254 has the 3-step cone structures254′, as shown in FIG. 9B.

Workpiece 250 with its photoresist 3-step cone structures 254′ is nowsubjected to baking at 150 degrees C. to melt photoresist 3-step conestructures 254′, as shown in FIG. 9C, creating lens-shaped regions ofphotoresist 254″ on fused silica workpiece 250.

Workpiece 250 with its lens-shaped regions of photoresist 254″ is nowsubjected to reactive ion etching which transfers lens shaped regions ofphotoresist 254″ into fused silica workpiece 250, as shown in FIG. 9D,creating array of micro lenses 256 on fused silica workpiece 250.Conditions for the reactive ion etch are 300 Watts, CF4 at 45 standardcubic centimeters per minute, O₂ at 5 standard cubic centimeters perminute, 200 mTorr for 40 minutes using a Phantom II reactive ion etchtool, from Trion Technology, Clearwater, Fla.

In another embodiment, a 3-D printer is used to deposit each layer ofdyed polymer for the photomask, as shown in FIG. 6A. In this case,layers 70 a′-70 e′ are each in the range from 1 micron to tens ofmicrons thick. In one embodiment, all layers have the same thickness andthe same transmittivity so regions with multiple layers havecorrespondingly reduced transmittivity depending on the number of layersthrough which an exposure beam travels. In another embodiment, differentlayers result in different reflectivities from a highly reflective masksubstrate depending on the thickness of the dyed polymer absorbing lightin both passes as it travels to and from the highly reflective masksubstrate.

The 3-D printer can be a standard ink-jet printer in which successivelayers of ink are deposited one after another. In one example, the dyeor pigment concentration in the ink or polymer binder is selected soeach layer of ink deposited has a transmittivity such as 80%. In anotherexample, each layer has a transmittivity of 65% so light penetratingfive layers would have its intensity diminished to 5% of the incidentintensity. In another example, the different print heads of the ink-jetprinter are used to print inks with different transmittivities so eachlayer can have its own transmittivity.

Other methods of additive manufacture, such as chemical vapor depositionthrough a mask or on a previously deposited patterned precursor layer onthe mask substrate, can be used to make multi-tone amplitude photomasks.Additive printing, such as with a nozzle that provides liquid polymerdroplets, can be used to form the light absorbing polymer steppedpattern, as shown in FIG. 11. Other metal, dielectric, or polymermaterials, two dimensional materials, such as graphene, and othermaterials with desired transmittivities or reflectivities can also beused.

While several embodiments, together with modifications thereof, havebeen described in detail herein and illustrated in the accompanyingdrawings, it will be evident that various further modifications arepossible without departing from the scope of the invention as defined inthe appended claims. Nothing in the above specification is intended tolimit the invention more narrowly than the appended claims. The examplesgiven are intended only to be illustrative rather than exclusive.

What is claimed is:
 1. A multi-tone amplitude photomask comprising: amask substrate having a pattern on a surface of the mask substrate,wherein the pattern defines a channel and includes a plurality of layersof material, wherein each layer of material provides a differentintensity of light when a light source shines light on the pattern. 2.The multi-tone amplitude photomask according to claim 1, wherein thenumber of layers of material is least three layers of material.
 3. Themulti-tone amplitude photomask according to claim 1, wherein the patternincludes a layer of a patterned polymer on the mask substrate andwherein the pattern in the polymer includes a first portion having afirst thickness and a second portion having a second thickness.
 4. Themulti-tone amplitude photomask according to claim 1, wherein the patternincludes a plurality of differently patterned layers of a polymer on themask substrate.
 5. The multi-tone amplitude photomask according to claim1, wherein the pattern includes: a first blanket layer of material onthe mask substrate; a second blanket layer of material on the firstblanket layer of material, wherein the first blanket layer and thesecond blanket layer arranged to form the pattern, wherein the firstblanket layer includes a first pattern and the second blanket layerincludes a second pattern different from the first pattern.
 6. Themulti-tone amplitude photomask according to claim 5, wherein the patternincludes a first region having none of the first layer and none of thesecond layer, a second region having the first layer and none of thesecond layer, and a third region having both the first layer and thesecond layer.
 7. The multi-tone amplitude photomask according to claim1, wherein the pattern on the surface of the mask substrate includes: atleast one material on the surface of the mask substrate; a first resistdisposed on the at least one material wherein a first opening in theresist extends completely through the first resist and wherein the firstopening includes a first region along the surface.
 8. The multi-toneamplitude photomask according to claim 7, wherein the pattern on thesurface of the mask substrate includes a first etched pattern in the atleast one material defined by the first opening wherein the first etchedpattern extends only part way through the at least one material andleaving a remaining portion of the at least one material.
 9. Themulti-tone amplitude photomask according to claim 8, wherein the patternon the surface of the mask substrate includes a second resist disposedon the at least one material, wherein a second opening in the secondresist extends completely through the second resist, and wherein thesecond opening extends above the first region.
 10. The multi-toneamplitude photomask according to claim 9, the pattern on the surface ofthe mask substrate includes a second etched pattern in the at least onematerial defined by the second opening, wherein the second etchedpattern extends at least partially through a remaining portion of the atleast one material to provide the pattern in the at least one materialon the mask substrate, and wherein the second etch pattern is aligned tothe first etch pattern.
 11. The multi-tone amplitude photomask accordingto claim 7, wherein the at least one material includes a first materialand a second material, wherein the first material has a first etchcharacteristic and the second material has a second etch characteristicthat is different from the first etch characteristic, wherein the firstand the second etch characteristics allow for etching all the waythrough the first material without substantially etching the secondmaterial, and etching all the way through the second material withoutsubstantially etching the first material.
 12. A multi-tone amplitudephotomask comprising: a mask substrate; and a channel on top of thesubstrate, wherein the channel is formed from a plurality of layers ofmaterial, and wherein each layer has an optical density that depends onthe material characteristic and layer thickness.
 13. A multi-toneamplitude photomask according to claim 12, wherein the channel hasrounded edges.
 14. A multi-tone amplitude photomask according to claim12, wherein a second channel is arranged above, and substantiallyorthogonal to, the channel.
 15. A multi-tone amplitude photomaskaccording to claim 12, wherein the channel has curved sidewalls.
 16. Amulti-tone amplitude photomask according to claim 12, wherein one of theplurality of layers of material is an etch stop layer.
 17. A multi-toneamplitude photomask according to claim 16, wherein the etch stop layerincludes a plurality of openings.
 18. A multi-tone amplitude photomaskaccording to claim 12, wherein the channel is sized and configured tocarry a liquid.
 19. A multi-tone amplitude photomask mask substratecomprising: a pattern on a surface of the mask substrate, wherein thepattern defines a channel and includes a plurality of layers ofmaterial, wherein each layer of material provides a different intensityof light when a light source shines light on the pattern.
 20. Amulti-tone amplitude photomask mask substrate according to claim 1,wherein the number of layers of material is least three layers ofmaterial.